The affected sib-pair possible triangle is covered by the IBD probabilities of two types of models, one with fixed mode of inheritance and general mating type frequencies, the other with varying mode of inheritance and random mating. In general, a point in the possible triangle corresponds to the IBD probabilities for many different modes of inheritance, thus it is inappropriate to estimate the IBD probabilities and solve for parameters such as penetrances and allele frequencies, unless one has knowledge of the mode of inheritance.